Thermal, flow measurement is based essentially on two measuring principles, thermal dispersion and thermal profile, or temperature rise. In the case of thermal dispersion, a heated measuring element is exposed to the flow of the measured medium. The cooling rate caused thereby is a measure for the flow velocity. In the case of thermal profile, or temperature rise, heat is introduced into a limited region of the flow, whereby the temperature is locally increased, from which, together with the supplied energy, the mass flow can in turn be calculated. In such case, two temperature sensors measure the temperatures of the measured medium at different points, most often before and after the supplied heat. A number of heating elements and temperature sensors are also put to use in order to obtain a better picture of the thermal profile.
In the case of both measuring methods, the sensors can be placed in the main line or in a bypass. The two principles overlap significantly in practice.
Conventional thermal, flow measuring devices for industrial processes usually use two temperature sensors embodied as equally as possible, which are arranged most often in pin-shaped, metal sleeves—so-called stingers—and which are in thermal contact with the medium flowing through a measuring tube or through the pipeline.
The two temperature sensors are usually installed in a measuring tube; the temperature sensors can, however, also be mounted directly in the pipeline. One of the two temperature sensors is a so-called active temperature sensor, which is heated by means of a heating unit. As heating unit, either an additional resistance heating unit is provided, or the temperature sensor is a resistance element, e.g. an RTD (Resistance Temperature Device) sensor, which is heated by conversion of an electrical power, e.g. by a corresponding variation of the electrical measuring current. More recently, so-called thin-film resistance elements, so called Thin Film Resistance Temperature Devices (TFRTD) have also been put to use. The second temperature sensor is a passive temperature sensor: It measures the temperature of the medium.
Usually, in a thermal, flow measuring device, the heatable temperature sensor is heated in such a manner, that a fixed temperature difference is set between the two temperature sensors. Alternatively, it has also been known to supply a constant heating power via a control unit.
If there is no flow in the measuring tube, then an amount of heat constant in time is required for maintaining the predetermined temperature difference. If, in contrast, the medium to be measured is moving, then the cooling of the heated temperature sensor is essentially dependent on the mass flow of the medium flowing past. Since the medium is colder than the heated temperature sensor, heat is transported away from the heated temperature sensor by the flowing medium. In order to thus maintain the fixed temperature difference between the two temperature sensors in the case of flowing medium, an increased heating power is required for the heated temperature sensor. The increased heating power is a measure for the mass flow of the medium through the pipeline.
If, in contrast, a constant heating power is fed in, the temperature difference between the two temperature sensors is lessened as a result of the flow of the medium. The particular temperature difference is then a measure for the mass flow of the medium through the pipeline or through the measuring tube.
There is thus a functional relationship between the heating energy needed for heating the temperature sensor and the mass flow through a pipeline or through a measuring tube. The dependence of the heat transfer coefficient on the mass flow of the medium through the measuring tube or through the pipeline is utilized in thermal, flow measuring devices for determining mass flow. Devices which operate according to this principle are available from the assignee under the designations, “t-switch”, “t-trend” or “t-mass”.
Thermal, flow measuring devices are especially suited for flow measurement of gases or gas mixtures.
Conventionally, the quantitative chemical composition of the measured medium must be known and must be configured in the measuring device. Thus, the mole fraction, volume fraction or mass fraction of each individual component of the measured medium, or parameters representing these, are input to the measuring device. In such case, gas mixtures with a plurality of components can be measured. In the case of applications with a variable composition for the fluid, measurement errors arise because the device calculates using fluid properties other than those actually present in the measuring tube at the point in time of measurement.
As is known to those skilled in the art, flow measurements with thermal, mass flow, measuring devices are, in general, dependent on the fluid measured medium. If the chemical composition of the measured medium, and therewith the heat transfer function of the measured medium, changes in the measuring device, the measuring device must be adjusted for this change in the chemical composition of the measured medium, i.e. specific parameters must be adjusted, in order to continue to measure the correct flow through the measuring tube.
Therefore, either the quantitative chemical composition of the measured medium is determined with a gas analysis method virtually continuously, or discretely with a relatively high measuring frequency, which is very complicated and expensive, or the quantitative chemical composition of the measured medium is estimated, and a measurement error of the thermal, flow measuring device is tolerated.